The capacity of a battery, such as a vehicle battery, may diminish over time due to, for example, over discharge on the battery or extreme heat. As a result, the battery may be unable to provide an amount of electrical energy necessary to start the vehicle and/or operate various vehicle electrical systems. Determining the capacity of a battery generally requires testing the battery while drawing a current. For example, one commonly accepted test for measuring the capability of a battery is to measure the amount of current a battery can provide at a given temperature for a fixed period of time while maintaining a set voltage. This measured current is generally referred to as cold cranking amps (CCA).
One conventional method for determining the capacity of a vehicle battery involves measuring the internal resistance or dynamic conductance of the battery. Measuring the internal resistance of the vehicle battery typically involves applying two different DC currents to the battery, calculating the difference in the voltage readings, and determining the internal resistance as a function of the different voltage readings. In some systems, the internal resistance of the battery is typically measured using a very high direct current for (e.g. 50 amps or more) to minimize the effect of changes in internal resistance due to change in the test current. Thereafter, the measured internal resistance or dynamic conductance values are compared to the size and state of the charge of the battery while it is under a current test to determine its condition. However, there are disadvantages associated with conventional systems that use high direct current. For example, high direct current methods typically require very bulky and expensive test equipment and require the battery to have enough charge in order to perform the test. In addition, such high direct current methods require the use of very high currents, which can temporarily alter the condition of the battery such that test results are not repeatable.
Another conventional method for determining the condition of a battery involves measuring the dynamic conductance/resistance of the battery by applying a time varying voltage, such as an alternating current voltage, and making periodic step transitions at a pre-determined frequency between two discrete voltage levels. Alternatively, the dynamic conductance/resistance of the battery can be measured by applying a time varying discharge current and making periodic transitions between two discrete current levels at a pre-determined frequency. At least one disadvantage with such method is that the dynamic conductance/resistance varies significantly with the state of charge, and it is difficult to measure the exact state of charge in batteries that are significantly discharged.